Composite coating of variable thickness having a gradient coloration in the cross-web direction

ABSTRACT

Composite coatings are provided having a variable thickness and a gradient coloration in the cross-web direction using a premetered extrusion coating process wherein a pigmented coating composition and a non-pigmented coating composition are simultaneously extruded onto a carrier film. The composite coatings are useful in preparing laminated structures such as automobile windshields with a colored gradient band.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to the preparation of composite coatingswhich provide a gradient coloration in the cross-web direction. Moreparticularly, a process for preparing a composite coating having avariable thickness in the cross-web direction is provided in whichpremetered coatings are simultaneously applied to a carrier film, onelayer of the coating being of a uniform thickness and another layerbeing colored and of variable thickness in the cross-web direction whenmeasured either from the center or an edge of the web. Coloration of thelayer of variable thickness provides a gradient in the optical densityof the composite which makes it suitable for use as an interlayer forproviding a vignette stripe in an automobile windshield.

A premetered coating apparatus consisting of dimensionally stable andchemically inert pieces forming a coating die may be modified inaccordance with the invention for use in preparing the novel coatings. Aflow channel for the coating fluid is formed by openings carved into thedie pieces, by openings formed when the die pieces are assembled, orboth. Typical practice is to configure the interior geometry of the dieso that coating fluid is extruded in a sheet of uniform thickness.

BRIEF DESCRIPTION OF PRIOR ART

It is known to provide a color image having a continuous gradation inthe cross-web direction by coating techniques such as gravure printing.Products having such a variation find utility in packaging applicationsas well as in safety glass for automobiles and in architecturalapplications.

In some applications, the gradated color stripe is formed by coextrudinga molten colored layer along with a clear thermoplastic sheeting. U.S.Pat. No. 4,316,868 describes one such process. In these processes, thecolored stripe is embedded in and forms an integral part of thethermoplastic sheeting.

Both of the just-mentioned processes have inherent limitations. Forexample, in applications such as gravure printing, the overall qualityof the gradated image is dependent on the quality of the engravedgravure cylinder. Applications which utilize pigment based coatingsolutions place additional limitations on gravure printing due toaccumulation of pigment particles under the doctor blade which is usedin the gravure process. This causes streaks in the coated product whichis unacceptable especially in applications requiring high opticalquality such as in automobile windshields. Further, limited life of thecylinder due to scratching and wearing of the cylinder surface by thedoctor blade requires re-engraving of the cylinders on a periodic basis,which adds additional expense. While colored gradations forthermoplastic sheeting for safety laminate applications by a coextrusionprocess overcomes the quality limitations inherent in gravure printing,flexibility with respect to colors and widths of the gradations islimited. Color and width changes require extensive cleaning of the dieand modifications to the die assembly which are costly in terms of downtime and labor requirements involved in the operations.

Blade, or knife coating, to provide a tapered cross-web profile isdisclosed in Australian published application AU-A-56153/90. In a bladecoating process, the wet coated thickness of a layer depends on thephysical properties of the coating solution such as viscosity. Precisecontrol of the coating thickness is especially difficult. Precisecontrol is said to be achieved in the Australian application by using acomplex pressure feed-back system. Optical density is said to becontrolled by mixing two solutions in different proportions. This canlead to nonuniformity of a gradient pattern.

Premetered coating processes such as slot coating have been used toproduce uniform coating thickness in the cross-web direction. U.S. Pat.No. 4,411,614 describes one such process wherein inserts in flowdistribution channels are used to improve the flow uniformity of coatingsolutions in the cross-web direction. Such processes have not been usedto provide coatings of variable thickness in the cross-web direction.

Simultaneous coating of more than one layer is also practiced inpre-metered coating processes. U.S. Pat. No. 2,901,770 describes onesuch apparatus.

The problems associated with the coating processes heretofore used aresolved by this invention which provides an easily controlled processwhereby a composite coating is deposited on a carrier film to provide anoptical density gradient coating of superior quality. The compositecoating can be readily transferred to an adhesive film such as polyvinylbutyral for incorporation in a laminated safety glass structure. Theprocess offers additional advantages since coatings having a variabledensity when measured either from the center or an edge of the web andcoatings of various widths may be produced prior to transfer. Further,in such a process, the coatings may be optically inspected prior totransfer and thus offer opportunities to minimize waste.

SUMMARY OF THE INVENTION

In the process of this invention a composite coating of variablethickness and a gradient coloration in the cross-web direction whenmeasured either from the center or an edge of the web is provided bydepositing coatings of a layer of a non-pigmented thermoplastic binderresin onto a carrier film and a colored coating of variable thickness inthe cross-web direction on top of said layer of non-pigmented binderresin. The coatings when applied as a solvent-containing solution aredried to remove the solvent and provide a solvent-free composite coatingon the carrier film. Coatings are provided which taper from a minimum toa maximum thickness from each edge of the film to the center of thefilm, or alternatively, from the center of the film to each edge of thefilm. The composite coatings can be optically inspected prior to beingslit to a selected width and incorporated in a laminated structure suchas being transferred to an adhesive interlayer sheet to provide alaminated glazing structure having an optical density gradient band.

The coatings are premetered through an extrusion die having a slotopening and distribution channels. In one embodiment, a distributionchannel is configured to provide a coating of variable thickness in thecross-web direction and another distribution channel is configured toprovide a coating of uniform thickness across the web. In a preferredembodiment internal shims are positioned in the distribution channel.The shims are shaped to deposit colored or pigmented coating in avariable thickness in the cross-web direction. The pigmented coatingmaterial is pumped through the top section of the extrusion die.Simultaneously, a non-pigmented coating material is pumped through thebottom section of the extrusion die at a rate to provide a coating ofuniform thickness. The two coating compositions are superimposed inlayers as they are extruded from the slot opening in the die.

Alternatively, the required thickness distribution of the coatings canbe obtained by configuring an extrusion die with the flow channel carvedin the walls of the die to direct the coating composition to therequired thickness distribution.

Included within the scope of this invention are variations of thosejust-mentioned in which additional layers of coating solution areapplied sequentially or simultaneously. Such additional layers may bepigmented or non-pigmented. These additional layers may have a uniformthickness or a gradated thickness.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a side elevation view of an extrusion coating die.

FIG. 2 is a side elevation of a dual-slot extrusion coating die.

FIG. 3 is a top view of shims which may be stacked together and insertedin the die of FIG. 1 or FIG. 2 to provide a cross-web thickness profileof greatest thickness at the edges of the coating.

FIG. 4 is a top view of shims which may be stacked together and insertedin the die of FIG. 1 or FIG. 2 to provide a cross-web thickness profileof greatest thickness at the center of the coating.

FIG. 5 is an optical density profile obtained using the stack of shimsshown in FIG. 3.

FIG. 6 is an Optical density profile obtained using the stack of shimsshown in FIG. 4.

FIGS. 7A, 7B and 7C are a schematic representation of an insert for adie cavity.

FIG. 8 is an optical density profile obtained with an insert of the typeshown in FIG. 7.

FIG. 9 is a section of one embodiment of the product of this inventionshowing the optical density profile of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, the coating apparatus shown is used in carryingout a premetered coating process. This type of coating process isdistinguished from other processes in that the rate of deposition ofcoating fluid is controlled by the rate at which the fluid is deliveredto the die. The extrusion die is formed from a top piece 10 and a bottompiece 12 fabricated from dimensionally stable and chemically inertmaterial. Stainless steel is typically used, but other suitablematerials may be used. A distribution channel is carved in the toppiece, the bottom piece, or in both pieces to distribute the coatingfluid across the width of the die. FIG. 1 shows a distribution channel12 carved in bottom piece 11. The coating fluid is formed into a sheetand directed to the web through an extrusion slot 14. Slot 14 is formedeither by spacers or shims between the die pieces or by an openingcarved into the pieces. In usual practice the combination of thedistribution channel geometry and the slot geometry cause the coatingfluid to deposit onto a web or carrier film as a sheet having a uniformthickness.

FIG. 2 shows the internal structure of a preferred apparatus of thisinvention. The extrusion die is formed by top piece 10, bottom piece 11and center piece 15 which separates the two coating layers. Distributionchannel 12 is carved in top piece 10 and bottom piece 11. Coating fluidis fed to the two distribution channels 12 by means of feed inlets 13.In some cases, the two distribution channels 12 along with the feedinlets 13 may be carved on either side of center piece 15. Extrusionslot 16 in the top layer is formed by a stack of shims such as 18, 19,20 or 21, 22, 23 illustrated in FIGS. 3 and 4, respectively. Asdescribed below, the said shims in the top layer are shaped to depositcolored or pigmented coating in a variable thickness in the cross-webdirection. Extrusion slot 16 may also be formed by an opening carvedinto top piece 10 or center piece 15. Extrusion slot 17 is formed eitherby shims between center piece 15 and bottom piece 11 or by an openingcarved into the said pieces.

In the design of interiors of coating dies, two basic approaches areused to insure a uniform distribution of coating solution. In the firstapproach, the distribution channel is large enough for the pressure dropin the distribution channel to be negligible relative to the pressuredrop in the slot. Since the variation in the entrance pressure to theslot is negligible, the variation in the flow rate through the slot isnegligible. In the second approach, the distribution channel is designedso that the sum of the pressure drop in the distribution channel and thepressure drop in the slot is uniform across the die width. In thepresent invention, these two approaches are extended to design dieinteriors so that there is a gradient in the flow rate in the cross-webdirection.

The embodiment shown in FIG. 7, utilizes an insert which may bepositioned in the distribution channel of the dies shown in FIGS. 1 and2 which changes the geometry of the channel. As shown in FIG. 7, theland length is made longer in regions 24 at the edge of the coating.There is no change in land length in region 25 at the center of thecoating. These changes in geometry cause more flow in the center of thecoating and less flow at the edges of the coating since greater landlength causes more restriction to flow. The present invention alsocovers inserts with land length variation in the central region of thecoating with no variation in the edges of the coating. This insert maybe fabricated from metal or a suitable polymeric material that can bemolded and will remain dimensionally stable under operating conditionsfor the die. In another embodiment, the insert is replaced by a shim ora stack of shims designed to provide the desired gradient. Parts ofthese shims fill previously open regions of the slot and alter theinterior geometry of the slot. Since the pressure drop of a Newtonianfluid flowing through a thin slot is proportional to the length of theslot and inversely proportional to the cube of the slot height, agradient in the flow rate of coating fluid is achieved.

The different shim design approaches of the current invention can beused to produce cross web coating thickness gradients. One approach,referred to as the edge taper approach is designed to produce coatingthickness gradients at the edges of the coating in the cross-webdirection. The top view as shown in FIG. 3 of the shape of theindividual shims 18, 19 and 20 which are stacked together to obtain edgetaper coating is schematically shown in FIG. 3. Another approach,referred to as the center taper approach is designed to provide coatingthickness gradients in the center of the coating in the cross-webdirection. As shown in FIG. 4, shims 21, 22 and 23 are stacked togetherto obtain coating thickness gradients in the center of the coating. Theshape of the individual shims and the number of shims stacked togethermay be changed, depending on the desired cross-web coating thicknessprofile, the properties of the coating fluid and the coating processconditions.

Coating thickness variation in the cross-web direction is found toresult in a corresponding cross-web optical density variation when thecoating contains colorants. FIG. 6 and FIG. 5 in the Example describedbelow shows optical density variations for the edge taper and centertaper approaches, respectively. For applications such as in automobilewindshields, the coating is slit in the central portion to result inpotentially two gradient bands. In some applications, it may beadvantageous to coat several sets of two or more coatings by appropriatedesign of the die or stack of shims or both.

FIG. 9 shows one embodiment of the product of this invention. Acomposite structure is provided consisting of a carrier film 30 having afirst layer of polymeric binder resin 28 and a colored polymeric binderresin layer 26 having a variable thickness and a gradient coloration(a-b and c-d) in the cross-web direction when measured either from thecenter or an edge of the web.

In a further embodiment of the invention, the interior geometry of thedie is machined to give the desired flow profile. FIG. 7 is a schematicrepresentation of a die insert used in a die cavity to obtain a flowvariation in the cross-web direction. This can be achieved byappropriate design of the distribution channel, extrusion slot, or both.

An advantage of the insert and especially of the shim designs is theeconomic advantage of providing a new and desired flow profile withoutthe expense of fabricating a new die. The specially designed die has theadvantage of production robustness.

This invention also encompasses coating multiple layers of coatingsolution either sequentially or simultaneously. In a preferredembodiment, a non-pigmented solution can be coated with a uniformcross-web thickness profile directly onto the web substrate and apigmented solution coated with a variable cross-web thickness profile ontop of the non-pigmented layer. There may be cases in which the reverseorder of layers may be preferable. Coating thickness of the two layersis determined by several factors such as optical density of the coloredlayer, cross-web coating thickness profile and the desired properties ofthe coating. Maximum dried coating thickness of the individual layers isabout 100 microns.

In carrying out the process of this invention, a pigmented coatingcomposition is prepared by dispersing colorant or pigments in a binderresin. A non-pigmented resin solution is prepared by dissolving thebinder resin in a suitable solvent or solvent blend. The resin selectedmay be the same or different from that used in preparing the pigmenteddispersion. Coating is accomplished at speeds from 9.14 meters perminute (30 feet per minute) to 152.4 meters per minute (500 feet perminute). The coatings are immediately dried after being deposited on thecarrier film.

Pigments used for coloration are preferably crystalline solids withextremely fine particle size having specific surface areas between 25and 600 square meters/gram as measured by the BET(Brunauer-Emmett-Teller) method. In selecting the pigments, colorstability is an important factor particularly in outdoor applicationssuch as automobile windshields. Light fast pigments such as copperphthalocyanine blue, copper phthalocyanine green, carbazole violet,anthraquinone red, quinacridone red, cadmium sulfoselenide red, monoazored, azo condensation yellow, monoarylide yellow, diarylide yellow aswell as carbon black or combinations of pigments may be used. In someapplications it may be advantageous to use a combination of pigments anddyes for achieving a balance of color stability and reduced haze.

In preparing the inks for use in this invention, suitable binder resinsinclude nitrocellulose, cellulose esters such as cellulose acetatebutyrate, cellulose acetate propionate and cellulose acetate, andpolyvinyl acetals such as polyvinyl butyral. Preferred binder resins arepolyvinyl butyrals having a hydroxyl content, calculated as polyvinylalcohol, from about 10 to 35% by weight. The polyvinyl acetate contentof these resins is from about 0 to 5% and the polyvinyl butyral contentis from about 60 to 90% by weight. The weight average molecular weightof these resins as determined by size exclusion chromatography is fromabout 10,000 to 250,000. The content of the polyvinyl alcohol, polyvinylacetate and the polyvinyl butyral along with the weight averagemolecular weight strongly influences various properties of the ink suchas surface tension, solvent/solvent blend selection, transferconditions, adhesion and shatter resistance of the film transfer coatingwhen used in safety glass applications. Preferred binder resin loadingsin the inks, expressed as weight percent of the binder resin in thecoating ink solution is about 0.1 to 40%.

The solvent or solvent blend should be chemically inert to the materialsused in the carrier film. Preferred solvents in amounts from about 28 to99% by weight of the ink which can be used are alcohols such asmethanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-butanol,diacetone alcohol and benzyl alcohol, glycol ethers such as1-methoxy-2-propanol, butyl glycol and methoxy butanol, esters such asglycolic acid-n-butyl ester, ketones such as cyclohexanone, andN-methyl-2-pyrrolidone. In addition non-solvents and solvents possessinglimited solubility such as methyl ethyl ketone, methyl iso-butyl ketone,methyl acetate, ethyl acetate, n-butyl acetate, aliphatic and aromatichydrocarbons such as cyclohexane and toluene, may be used in conjunctionwith solvents.

Dispersants are useful in preparing the pigment based inks used in thisinvention. The choice of dispersant will depend on the pigment, binderresin and the solvent used in the inks. Additives may be used in theinks and the binder resin coatings to enhance the flexibility of thecoatings such as plasticizers. In addition, non-ionic surfactants may beused to reduce surface tension of the ink and to aid in wetting andleveling of the coating on the carrier film.

The carrier film may be selected from such materials as polypropylene,polyester, polyamide and polyvinyl fluoride films. The thickness of thefilm is generally about 0.00127 to 0.0762 centimeters (0.0005-0.03inch). The carrier film may be treated to a desired surface tensionlevel through flame treatment or corona treatment which is well known tothose skilled in the art.

The products of this invention are particularly useful for thermaltransfer to thermoplastic sheeting having a roughshed surface, such asplasticized polyvinyl butyral used in safety glass laminates. Thecomposite structure transfers readily to the roughened surface atmoderate temperatures by passing the polyvinyl butyral sheeting and thecomposite in contact with each other through nip rolls heated to -1° to150° C. (30° to 300° F.). By providing a non-pigmented coating ofuniform thickness across the carrier film, the pigmented coating whichvaries from a maximum thickness to essentially zero is supported duringthe transfer, and the quality of the optical gradient is not disturbedin the transfer process.

The products of this invention may be used directly in preparinglaminates. For example, a glass/plastic laminate may be prepared byadhering the composite to a glass sheet using a suitable known adhesivematerials such as polyvinyl butyral or polyurethane film. The carrierfilm may or may not be coated with a known antiabrasion coating of thetype disclosed in U.S. Pat. Nos. 4,177,315 and 4,469,743.

The invention is further illustrated by the following examples in whichparts and percentages are by weight unless otherwise indicated.

EXAMPLE 1

The stack of shims illustrated in FIG. 3 and FIG. 4 were inserted in anextrusion die as shown in FIG. 2 and used to coat edge and center tapercoatings respectively. The shims were used in the assembly of the toplayer of a two layer extrusion die. The die for the bottom layer wasdesigned to coat a uniform thickness of the coating solution in thecross-web direction. A pigmented solution was applied through the toplayer of the die and a non-pigmented solution was applied through thebottom layer of the die. The following are the compositions of theindividual layers:

    ______________________________________                                        Pigmented solution:                                                           Carbon black       0.90%                                                      Polyvinylbutyral resin                                                                           13.10%                                                     1-propanol         38.70%                                                     1-methoxy-2-propanol                                                                             38.70%                                                     n-methyl-2-pyrrolidone                                                                           8.60%                                                      Non-pigmented solution:                                                       Polyvinylbutyral resin                                                                           9.81%                                                      1-propanol         60.08%                                                     1-methoxy-2-propanol                                                                             24.71%                                                     n-methyl-2-pyrrolidone                                                                           5.45%                                                      ______________________________________                                    

The coatings were applied on a 0.0051 centimeter (0.002 inch) thickpolyethylene terephthalate film at a speed of 45.72 meters per minute(150 feet per minute). The coating width was 30.48 centimeters (12inches). After application of the coating solution, the film was passedthrough an air dryer set at 66° C. (150° F.) to remove the solvents. Theaverage coating thickness of the dried composite coating was 2 microns.The coating thickness of the composite coatings was determined in thecentral portion of the edge taper approach and edge portion of thecenter taper approach since the coating thickness, which ischaracterized by constant optical density, is essentially constant asshown in the optical density profiles in FIG. 5 and FIG. 6 discussedbelow. Optical density was determined in equal increments over the 30.48centimeter width of the coating with a transmission densitometer (ModelTD-904, Macbeth, A division of Kollmorgen Corporation, Newburgh, N.Y.).For example, the position number shown in FIG. 6 has been divided into95 equal segments which are spaced 0.3208 centimeters (0.1263 inches)from each other.

As shown in FIG. 6, the optical density as a function of position on theweb varies from nearly zero at the edges corresponding to nearly zerocoating thickness of the pigmented layer to a maximum value at thecenter corresponding to the darkest area of the band. In the centertaper bands, the optical density profile on FIG. 5 varies from nearlyzero at the center corresponding to nearly zero coating thickness of thepigmented layer to a maximum value at the edges, corresponding tomaximum coating thickness. To form the stripe required for a vehiclewindshield, the coated band is slit down the center to yield two stripepatterns.

EXAMPLE 2

A coating was made with the die insert illustrated in FIG. 7. Thisinsert was placed in the cavity of a single layer coating die. Theinsert was designed to produce edge taper coating by directing more ofthe coating flow to the center of the die and relatively less of theflow to the edges of the die. The solution composition for this coatingis given in U.S. Pat. No. 4,489,154, "Process for Preparing a SurprintProof" to H. W. Taylor

The coatings were applied at a speed of 15.24 meters per minute (50 feetper minute) to a 0.0051 centimeter (0.002 inch) thick polyester film.The coating width was 30.48 centimeters (12 inches). After applicationof the coating, the web was passed through an air drier set at 77° C.(170° F.) to volatilize the solvent. The corresponding optical densityprofile of the final film was determined as described in Example 1 anddisplayed in FIG. 8.

The above example illustrates that coating thickness variation in thecross-web direction can be achieved by appropriate design of the cavityof a die.

What is claimed is:
 1. A process for preparing a composite coating ofvariable thickness and a gradient coloration in the cross-web directioncomprising depositing onto a carrier film,(a) a layer of polymericbinder resin solution having a uniform thickness across the width of thefilm and a layer of colored polymeric binder resin of variable thicknessin the cross-web direction and having a gradient coloration in thecross-web direction when measured either from the center or the edge ofthe web, said variable thickness tapering from a minimum to a maximumfrom each edge to the center of said film or from a maximum to a minimumfrom each edge to the center of said film, and (b) drying the coatingsto provide a dry composite coating, (c) said individual layers having amaximum dried coating thickness of about 100 microns.
 2. The process ofclaim 1 wherein said solutions are premetered through an extrusion die(10, 11, 12, 13) having a slot opening (14), said die being configuredto provide a flow rate gradient for said colored binder resin solution(26) through said slot opening (14) in the cross-web direction.
 3. Theprocess of claim 1 wherein the said thickness gradient (a-b, c-d) isprovided by a distribution channel (12) having internal shims (18, 19,20) positioned in said distribution channel, said shims being configuredto provide a predetermined flow rate gradient in the cross-webdirection.
 4. The process of claim 1 wherein the said colored coatingsolution (26) contains pigments.
 5. The process of claim 1 wherein saidpolymeric resin is polyvinyl butyral.
 6. The process of claim 1 whereinthe surface of the said composite coating is coated with a layer ofnon-pigmented binder resin solution.
 7. The process of claim 1 whereinthe layers are deposited simultaneously.
 8. A carrier film having acomposite coating thereon comprised of a layer of polymeric binder resinhaving a uniform thickness and a colored polymeric binder resin layerhaving a variable thickness and a gradient coloration in the cross-webdirection when measured either from the center or an edge of the web,said variable thickness tapering from a minimum to a maximum from eachedge to the center of said web, said individual layers having a maximumdried coating thickness of about 100 microns.
 9. The product of claim 8wherein said binder resin is polyvinyl butyral.
 10. The product of claim8 wherein said colorants are pigments.
 11. The product of claim 8wherein said film is selected from the group consisting of polyester,polyproplyene, polyamide and polyvinyl fluoride films.